The present invention relates to optical beam scanning apparatus and, more particularly, to a wobble compensated beam scanning apparatus having a rotatable beam deflector with at least one reflective surface that lies in a plane which intersects the axis of rotation at an acute angle.
Optical beam scanners having a rotating, pyramidal mirror for scanning a writing or reading beam across a media-containing image/object plane are well known in the art. Representative examples of such pyramidal mirror scanning systems are found in a variety of the now ubiquitous "laser printers" and "laser scanners". The pyramidal mirrors used in such printers and scanners can be divided into two major categories: one in which the planes of the reflecting surfaces are parallel to the rotational axis of the pyramidal mirror; and, two, those in which the planes of the reflecting surfaces of the pyramidal mirror intersect the mirrors' rotation axis at an acute angle.
The quality of the output media from the printers and scanners depends in a large measure upon the positional accuracy of the scanning beam. In order to maintain the desired degree of beam placement accuracy, relative tight positional requirements are imposed upon both the reflecting surfaces of the pyramidal mirror and the rotation axis of the mirror.
The rotation axis of the pyramidal mirrors in both categories must remain "true" with respect to the optical system. If the rotation axis departs from the "true" rotation axis, a cross-scan error is introduced in the optical system. The departure of the mirrors' rotation axis from the "true" rotation axis is caused by "wobble" in the scanning apparatus. The wobble can be either random or fixed or a combination thereof. Random wobble is introduced by loose spin motor bearings, non-spherical ball bearings and the like, while the fixed wobble is introduced when the rotation axis of the pyramidal mirror is misaligned with respect to the "true" rotation axis. The wobble induced cross-scan deviation must be compensated for in order to achieve a high degree of addressability in the image/object plane.
One embodiment of the present invention is directed to wobble compensated beam scanners employing pyramidal mirrors of the second category in which the planes of the reflective scanning segments intersect the axis of rotation at an acute angle.
The use of a rotating, pyramidal scanner is old in the art. See U.S. Pat. No. 3,487,224, issued Dec. 30, 1969 for a "Scanner Which Utilizes A Pair Of Time-Shared Apertures". Truncation of the rotating pyramidal reflective scanning element is shown in the following U.S. Pat. No.: 3,866,038, issued Feb. 11, 1975 for "Apparatus For Measuring Surface Flatness"; U.S. Pat. No. 4,268,110, issued May 19, 1981 for "Facet Angle Corrector For Multi-Faceted Optical Scanner"; U.S. Pat. No. 4,312,590 issued Jan. 26, 1982 for "Optical Scanner And System For Laser Beam Exposure Of Photo Surfaces"; and, U.S. Pat. No. 4,323,307 issued Apr. 6, 1982 for "Light Beam Scanning Apparatus".
The 4,312,590 and 4,323,307 patents disclose a wobble compensated scanner utilizing a rotating, truncated pyramidal reflector with two roof mirrors. The truncated portion of the pyramidal reflector is not employed either as a reflecting surface or as an active element in the optical system. In each patent, wobble compensation is achieved only through multiple reflections between the rotating, truncated pyramidal mirror sides and the roof mirrors.
It is accordingly a general object of the present invention to provide an improved wobble compensating beam scanner.
It is a specific object of the invention to provide a wobble compensating beam scanner having at least one reflective surface that lies in a plane that intersects the axis of rotation at an acute angle.
It is another object of the invention to provide a beam scanner utilizing a rotating, truncated pyramidal mirror with the truncation surface acting as a wobble compensating optical element in the scanner.
It is still another object of the present invention to provide a wobble compensated beam scanning apparatus having a compact form factor for use in laser beam printers and scanners.
It is a feature of the invention that multiple scans of the read/write media can be achieved during a single rotation of the wobble compensated rotating, truncated pyramidal mirror.
It is another feature of the invention that the reflecting characteristics of the reflecting surfaces of the pyramidal mirror can be varied to accommodate intensity variations in the laser beam light source.